Shoppers kicked off the season of buying with a strong Black Friday weekend, a signal that the economy's critical retail sector is gaining momentum.

Consumers spent 13 percent more over the weekend than in 2011, the National Retail Federation said. More people shopped, more people shopped earlier in the week, and more people said they would shop online on "Cyber Monday."

Roughly 139 million Americans spent $59 billion in stores and online from Thursday to Sunday, the retail group estimated. Another 129 million were expected to buy stuff online on Monday.

In an economy roughly 70 percent driven by consumer spending, the holiday shopping season has come to be perceived as crucial.

Economists are mixed on how useful Thanksgiving weekend is in predicting spending over the following month. But for retailers, concepts like Black Friday and Cyber Monday take on outsized importance. They offer one way -- however imperfect -- to assess consumers' willingness to buy as the holidays approach.

The long lines that started forming Thursday night and sent swarms of shoppers into the aisles through the weekend signal that consumers have gained confidence as the economy recovers from the recession, said Lorman Lundsten, a business professor at the University of St. Thomas.

"We find people to be quite optimistic, and I consider that to be a barometer for people expecting this recession to be over with," he said. "The holiday season is a stimulus for spending, and it seems that this year it's going to be quite useful."

Not everyone is persuaded of Black Friday's importance, however.

Paul Dales, an analyst for Capital Economics, advises people not to read too much into the hustle-bustle of the past few days.

If anything, a strong Black Friday tends to predict a slightly weaker holiday shopping season overall, he said. Rational consumers may have bought more over the past weekend, but that doesn't mean they plan to dig deeper into their pockets.

"History suggests that strong sales on Black Friday tend to be followed by weak sales over the rest of the holidays, and that weak sales on Black Friday tend to be followed by strong sales later on," Dales wrote in a note to investors.

Joel Waldfogel, an economist at the Carlson School of Management, is skeptical of the Black Friday phenomenon. He estimates that holiday-specific spending is a fraction of the figure the trade group publishes.

Based on comparisons of monthly retail sales figures, Waldfogel suggests holiday spending in 2011 was less than a fifth of the National Retail Federation's figures. The discrepancy arises from the fact that people shop -- and spend -- every weekend of the year.

Yes, consumers spent billions of dollars in stores over the past weekend, Waldfogel said, but they also did so on the third weekend in June, and in July and August.

"On the one hand you see this ton of spending that happens on Thanksgiving weekend," he said. "But most of that would have occurred anyway."

Adam Belz • 612-673-4405 Twitter: @adambelz